Vitamin D deficiency

Vitamin D deficiency is defined as a serum 25-hydroxyvitamin D level < 20 ng/mL and 21-29 ng/mL is considered insufficiency.

The pathophysiology of vitamin D deficiency primarily involves inadequate cutaneous synthesis due to decreased exposure to sunlight, leading to impaired intestinal absorption of calcium and secondary hyperparathyroidism. This can result in increased bone turnover and defective mineralization of the collagenous matrix. Other mechanisms of deficiency include inadequate dietary intake, reduced gastrointestinal absorption in gastrointestinal disorders, impaired hepatic 25-hydroxylation due to certain medications or liver disease, increased metabolism of 25-hydroxyvitamin D in cancer, granulomatous diseases, or primary hyperparathyroidism, and rarely, loss of vitamin D-binding protein due to nephrotic syndrome or peritoneal dialysis.

The prevalence of vitamin D deficiency varies significantly by geographic region, population group, and age. The prevalence of vitamin D deficiency in the US is estimated at 24.6%.

Risk factors for vitamin D deficiency include inadequate sunlight exposure, malabsorption syndromes, bariatric procedures, renal or hepatic disease, cancer, autoimmune diseases, and certain medications such as anticonvulsants and glucocorticoids. People with dark skin, residing at higher latitudes (> 33° north or south), older adults, pregnant individuals, and patients with obesity are at higher risk.

The clinical course of vitamin D deficiency can range from asymptomatic to severe, depending on the extent of the deficiency and the presence of comorbid conditions. Symptoms can include diffuse musculoskeletal pain, bone pain, and muscle weakness. In severe cases, it can lead to secondary hyperparathyroidism, osteopenia, osteomalacia, and bone fractures in adults and rickets in children. Vitamin D deficiency has also been associated with an increased risk of CVDs, T2DM, preeclampsia, infections, and cognitive impairment.

The prognosis of vitamin D deficiency is generally good with appropriate treatment, which typically involves supplementation with vitamin D3. However, untreated deficiency can result in long-term skeletal and extra-skeletal consequences and has been linked to an increased risk of mortality.